Carbon deposition boundaries and other constant parameter curves, in the triangular representation of CHO equilibria, with applications to fuel cells

In the CHO system, at pressures of the order of 1 atm and temperatures between 400°K and 1500°K, the only important species present at true thermodynamic equilibrium are the simple molecules: C, CH₄, CO, CO₂, H₂ and H₂O (apart from excess O₂).

This has recently been shown by Cairns, Tevebaugh and Holm [2], [3], who also introduced the very useful triangular representation of carbon deposition data in CHO diagrams.

It will be shown in this communication that carbon deposition boundaries and other curves of constant parameter values can be computed directly from three independent equations involving three equilibrium constants, and the balance equation: σ partial pressures = total pressure. No material balance equations and no trial and error procedures are needed, while the mathematical “problems” are confined to the solution of a quadratic equation as the most involved step.

Apart from carbon deposition boundaries, CH₄ and H₂O “isobars” and curves of constant reversible oxidation potential in fuel cells with O⁼ transport and CO₃⁼ transport have been computed for temperatures of usual interest. The method has been outlined for each of these cases.

Some theoretical consequences of the trend for p(H₂O) to remain constant in low or intermediate temperature fuel cells with aqueous electrolyte, have been pointed out in connection with the possibility of C deposition on highly active fuel electrodes.

An example pertinent to the use of the diagrams shown, is given in Appendix 2, where also the calculation of the complete equilibrium composition from CH₄ isobars is discussed.